A condition that is caused by hyperplasia of lymphocytes in the small intestine (intestine, small) and the mesenteric lymph nodes. These lymphocytes produce an anomalous alpha heavy chain protein. Generally, these IPSID patients have either concurrent lymphoma or develop lymphoma within a few years. The disease was first described in the mediterranean region and is characterized by malabsorption; weight loss; diarrhea; and steatorrhea.